1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus. More particularly, the present invention relates to a developing device with the use of a two-component developer including a toner and a magnetic carrier, and an image forming apparatus such as an electrostatic copying machine, a laser printer and a facsimile machine that form images using the developing device by an electrophotographic method.
2. Description of the Related Art
In the image forming apparatus using the electrophotographic method, an electrostatic latent image is formed on the surface of a photoconductor drum (toner image holder), a toner is supplied to the photoconductor drum by means of a developing device to develop the electrostatic latent image, a toner image formed on the photoconductor drum through the development is transferred onto a sheet such as a paper sheet, and the toner image is fused onto the sheet by means of a fuser.
In recent years, for a full-color compliant and high-definition compliant image forming apparatus, a two-component developer (hereinafter, may be referred to simply as “developer”) has been widely used, which is excellent in toner charging stability. The developer includes a toner and a magnetic carrier. The toner and the carrier are agitated in a developing device to generate friction between the toner and the magnetic carrier, and the friction allows the toner to be appropriately charged.
The charged toner is supplied to a surface of a developer holder such as a developing roller. The toner supplied to the developing roller is transferred to an electrostatic latent image formed on a photoconductor drum by electrostatic attraction. Thus, a toner image based on the electrostatic latent image is formed on the photoconductor drum.
Further, such an image forming apparatus is required to be more high-speed and downsized. It is therefore necessary to quickly perform sufficient charging of a developer and to quickly convey the developer.
To this end, as a today's image forming apparatus, there has been proposed an image forming apparatus including a circulative developing device in order to instantly disperse supplementary toner into a developer to give an appropriate amount of charge.
The circulative developing device, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-255723, for example, has two conveyance paths and two conveyance screws in a developer case, in which the conveyance paths allow a two-component developer to be conveyed and circulated therethrough, and the conveyance screws are respectively disposed in the conveyance paths to simultaneously agitate and convey the two-component developer.
The developer is conveyed toward one end by one of the conveyance screws (second conveyance screw), and then conveyed toward the other end by the other conveyance screw (first conveyance screw) to be circulated under agitation. The first and second conveyance screws are configured to have different numbers of screw spirals to improve the dispersibility of the developer in a direction toward a developing roller and obtain stable and good image quality.
In addition, Japanese Unexamined Patent Application Publication No. 2009-109741 proposes a developing device including, in a housing for containing a developer, a first agitation and conveyance screw, a second agitation and conveyance screw, and a developing roller, wherein the first and second agitation and conveyance screws each have a helical blade and a reverse helical blade having a phase (pitch) moving reversely to the helical blade, and the reverse helical blade is disposed at an downstream side end of the screw in the developer conveyance direction and has a through hole.
In this developing device, each reverse helical blade reversely conveys the developer and returns the developer through the through hole to generate convection in the developer near the reverse helical blade, so that the developer in the dead space is agitated efficiently and generation of a pool of the developer is prevented.
In the above-described conventional developing device, however, the developer turns at a right angle at a downstream side end of one conveyance screw to move toward an upstream side end of the other conveyance screw. Upon the turning, some developer fails to turn at a right angle and has nowhere else to go to stay at the downstream side end.
Since the developer is pushed only in a rotation axis direction by rotation of the conveyance screw, the developer conveyed to the downstream side end of the conveyance path and the developer, having nowhere else to go, stays at an inner wall of the housing of the developer tank or just before the reverse helical blades push against each other, and as a result, the pressure on the developer is rapidly increased.
Once the pressure is increased, the developer at the downstream side end will continue to receive the pushing force (stress) by the conveyance screw in the rotation axis direction, while the velocity component of the developer in the rotation axis direction is zero.
That is, the developer staying at the inner wall of the housing or just before the reverse helical blade will receive a shear force, while being compressed.
As a result, a toner flow improver included in the toner as an external additive is buried in the toner surface due to heat generated by the stress and the shear force to significantly reduce the flowability of the developer, causing a lowered image density.
That is, when the developer is subjected to stress, the flow improver buried in the toner surface reduces the flowability of the developer to hinder the conveyance of the developer itself. As a result, the photoconductor drum is prevented from being supplied with a sufficient amount of developer via the developing roller to lead to a lowered density of an image printed on a recording medium.